An ethylene-vinyl alcohol copolymer, which henceforth may be abbreviated as EVOH, is a resin excellent in gas barrier properties, but it cannot be used for some applications due to its insufficient flexibility. Therefore, improvement in flexibility has been attempted through modification of EVOH or blending of soft resins to EVOH.
Patent Reference 1 discloses a resin composition comprising EVOH and a thermoplastic resin having a boronic acid group or a boron-containing group capable of being converted into a boronic acid group in the presence of water. As such a thermoplastic resin having a boron-containing group, soft resins such as an ethylene-propylene copolymer and a hydrogenated styrene-isoprene-styrene block copolymer are also used. It is disclosed that incorporation of such soft resins improves the impact resistance of EVOH. However, the flex resistance and impact resistance at low temperature are still insufficient and therefore in some cases such products cannot be used in applications where flexing or impact is applied at low temperature.
Patent Reference 2 discloses a modified EVOH comprising 0.3 to 40 mol % of structural unit (I) shown below and having an ethylene content of 5 to 55 mol %. This modified EVOH is superior in barrier properties, transparency, stretchability, flexibility and flex resistance and is used in the form of a monolayer or multilayer structure for various types of molded articles.
wherein R1, R2, R3 and R4 denote a hydrogen atom, an aliphatic hydrocarbon group having 1 to 10 carbon atoms, an alicyclic hydrocarbon group having 3 to 10 carbon atoms or an aromatic hydrocarbon group having 6 to 10 carbon atoms; R1, R2, R3 and R4 may be the same group or may differ; R3 and R4 may be combined together; and R1, R2, R3 and R4 may have a hydroxyl group, a carboxyl group or a halogen atom.
Furthermore, Patent Reference 3 discloses a resin composition comprising the modified EVOH disclosed in Patent Reference 2 and another thermoplastic resin. Examples in which various thermoplastic resins are used as the another thermoplastic resin are disclosed. It is disclosed that a compatibilizer is preferably used in addition when polyolefin or polystyrene is used as the another thermoplastic resin. Substances modified with maleic anhydride or substances having boronic acid (ester) groups introduced are disclosed as examples of such a compatibilizer. In working examples, examples where EVOH and polyolefin are blended by using a VLDPE or hydrogenated styrene-butadiene-styrene block copolymer modified with a boronic acid ester as a compatibilizer are disclosed. However, the flex resistance and impact resistance at low temperature were insufficient.
In addition, Patent Reference 3 also discloses a resin composition obtained by mixing a modified EVOH disclosed in Patent Reference 2, a crosslinking agent and an elastomer having a functional group which can react with the crosslinking agent under melting conditions, thereby conducting dynamic crosslinking treatment. As such an elastomer, a hydrogenated product of a styrene-butadiene-styrene block copolymer modified with maleic anhydride and a hydrogenated product of a styrene-butadiene-styrene block copolymer having boronic acid ester groups are disclosed. When, however, dynamically crosslinking it while kneading, it is impossible to make dispersed particles small enough and, as a result, the flex resistance and impact resistance at low temperature are insufficient.
Patent Reference 1: JP 06-340783 A
Patent Reference 2: WO 02/092643 A1
Patent Reference 3: WO 03/072653 A1